The present invention generally relates to a tone arm assembly of the type employed with sound reproduction equipment, such as phonograph assemblies and the like. In particular, the present invention is directed to a uniquely balanced tone arm assembly capable of distortion free tracking across even highly warped record surfaces.
Conventional pivoting-type tone arm assemblies usually include an elongated arm member having an intermediate portion pivotally attached to a support housing mounted on a frame portion of the phonograph assembly. A pick-up cartridge and stylus assembly is attached to one end portion of the elongated arm, with a counterweight adjustably mounted on the opposite end portion. By pivotally attaching the arm to the housing assembly, it is possible for the stylus to move in both the vertical and horizontal directions as required when tracking over an undulating record surface. In order to achieve high fidelity sound reproduction, it is essential that the stylus be allowed to precisely follow each of the peaks and troughs formed in the record surface. If the stylus is deflected from its normal tracking path across the record surface, a degree of distortion is introduced, with the amount of distortion being directly dependent on the amount of deflection incurred by the stylus. The problem facing the prior art is to pivotally balance the tone arm above the record surface in such a manner that the stylus is allowed to move as freely as possible over the peaks and troughs of the record surface.
Early pivoting-type tone arm support assemblies usually employed independent bearing systems for controlling the vertical and lateral movements of the tone arm relative to the record surface. For example, ball races, knife edges and even simple sleeved members often made up one or both of the independent bearing systems. Such multiple bearing systems have proven less than completely satisfactory, in that it is practically impossible to isolate the bearing systems from affecting one another. As a result, the bearing assembly controlling lateral movement tended to inhibit free vertical movement of the tone arm, while the vertical bearing assembly tended to somewhat inhibit the lateral movement of the tone arm, resulting in an overall reduction in the fidelity of the sound reproduced from the record. Furthermore, because such independent bearing assemblies were generally complex in structure, it was difficult, if not impossible, to maintain the tone arm in a balanced condition over an extended period of time.
In an effort to overcome the type of problems associated with multiple support bearing assemblies, it has been suggested that the entire tone arm assembly be balanced on a single, unipivot bearing assembly capable of pivoting the tone arm in both the vertical and the lateral direction relative to the record surface. In order to balance the tone arm above the support housing, the prior art considered it essential that the pivot point of the unipivot bearing assembly be positioned vertically above the center of gravity of the tone arm to create a so-called beam-balance mounting assembly. Such an arrangement was considered to be stable in both the vertical and horizontal planes, in that any pivoting movement of the tone arm would generate a restoring force which tended to restore the tone arm to its initially balanced position. In effect, pivoting movement of the tone arm tended to displace the center of gravity of the tone arm assembly toward the raised end portion, thereby creating a restoring force which attempted to return the center of gravity to its initial position. The main drawback of such beam-balance mounting systems is that the restoring forces generated during the vertical pivoting movement of the tone arm opposed the vertical tracking movement of the stylus, thereby deflecting the stylus from its normal path. As a result of such deflections, the sound reproduced from the record was distorted with a corresponding reduction in the fidelity of the tone arm assembly.
The beam-balance achieved by positioning the pivot point vertically above the center of gravity of the tone arm did provide some sideways or lateral stability to the tone arm. In particular, lateral rolling movement of the tone arm generated restoring forces which tended to return the tone arm toward its proper position.
Typical of such a beam-balanced unipivot tone arm is the assembly suggested in U.S. Pat. No. 3,231,283 issued Jan. 25, 1966 to Stanton. Stanton clearly suggested that the pivot point be positioned vertically above a plane passing through the center of gravity of the tone arm, the plane being defined as extending through the center of gravity of the counterweight and the head shell, respectively. Because a beam-balance effect is created in Stanton, longitudinal pivoting movement and/or lateral rolling movement of the tone arm must create restoring forces proportional to the degree of movement. These restoring forces attempt to return the tone arm to its initial position and thus adversely affect the desired tracking force acting on the stylus. As the tone arm in Stanton tracks over a warped record surface, the relatively large vertical pivoting movement of the tone arm creates a significant restoring force which opposes any movement of the tone arm away from its initial position. While Stanton provides desirable lateral balancing of its tone arm, the adverse restoring forces generated during vertical tracking movements of the tone arm render the Stanton assembly totally unsatisfactory.
Applicant has determined that if the pivot point of a unipivot bearing assembly can be carefully positioned to coincide with an imaginary horizontal line intersecting the center of gravity of the tone arm sub-assembly and extending perpendicular to an axis joining the head shell with a rear counterweight, vertical tracking movements of such a tone arm will not generate unwanted restoring forces. As the tone arm vertically pivots while tracking over a record surface, the only vertical force acting on the tone arm is the tracking force selectively chosen through placement of the rear counterweight.
In a further approach to the problem of providing proper balance, U.S. Pat. No. 4,113,265 issued Sept. 12, 1978 to Iyeta suggests employing a pair of lateral counterweights for controlling sideways or lateral roll of the tone arm. the counterweights are located forward of the pivot point in Iyeta and tend to significantly increase the mass of the tone arm assembly. Furthermore, because the inertia of the tone arm is equal to the mass times the square of the distance from the pivot point, the forward mounted counterweights suggested in Iyeta tend to generate an excessive inertia leval. Another drawback is the lateral counterweight induced rotation of the tone arm about its longitudinal axis as the Iyeta tone arm tracks in a vertical direction on a warped record. This phenomenon produces uneven record and stylus wear as well as reduced channel separation between the left and right signals with a resulting decrease of stereo effect from the loud speakers.
In order to provide a tone arm with sufficient lateral stability and yet eliminate the types of problems confronting Iyeta, applicant has determined that a pair of laterally arranged counterweights should have a center of gravity disposed vertically below the pivot point and the center of gravity of the tone arm sub-assembly. Furthermore, to prevent the tone arm from rolling when tracking warped records, the axis joining the counterweights should extend perpendicular to the stylus cantilever. Because the center of gravity of the counterweights is slightly below the pivot point, very slight restoring forces are generated during vertical movement of the tone arm. For example, in one embodiment restoring forces in the range of 1-5 milligrams (mg) were generated for a 1/4 inch vertical movement of the tone arm. Such restoring forces can be considered inconsequential when compared to conventional tracking forces measured in grams. In effect, because applicant's specially constructed lateral counterweights provide the tone arm with lateral stability with a minimum increase of inertia, the pivot point essentially coincides with the center of gravity to effectively eliminate vertical restoring forces which plague conventional tone arm assemblies of the Stanton type.
As will become evident from a reading of the following specification and claims, the tone arm assembly of the present invention also includes additional structural features which further reduce the amount of distortion introduced into the sound reproduction process. For example, the tone arm itself is formed from a pair of concentrically extending tubular members having a plurality of spaced, acoustic dampening bands positioned therebetween to reduce resonances from being transmitted through the tone arm.